The present invention relates to improved additives for preventing fluid loss from cement.
Cement compositions are used in the oil and gas industry to cement the annular space in the wellbore between the surrounding formation and the pipe or casing. A cement slurry typically is pumped down the inside of the casing and back up the outside of the casing through the annular space. The amount of water used in forming the cement slurry will vary depending upon the type of hydraulic cement selected and on the relevant job conditions. The amount of water used can vary over a wide range, depending upon such factors as the required consistency of the slurry and upon the strength requirement for a particular job.
Many times, the hydraulic cement must be placed within or next to a porous medium, for example earthen strata in the wellbore. When this happens, water tends to filter out of the slurry and into the strata during placement and setting of the cement. Many difficulties are related to an uncontrolled fluid loss of this type. Such difficulties include an uncontrolled setting rate, improper placement of the slurry, impaired strength properties, and contamination of the surrounding strata. These conditions are all undesirable in oil and gas well cementing operations.
In order to lessen the fluid loss from the aqueous cement slurry, various materials have been employed in the past. One such material is a copolymer of an acrylamide and acrylic acid. This fluid loss material has the ability to maintain fluid loss control over a wide temperature range, functions well in the presence of salts, and is superior to cellulose base additives when chloride salt accelerators are present.
The effectiveness of a fluid loss additive is related to the size or the molecular weight of the polymer. A xe2x80x9clargexe2x80x9d polymer, or a polymer with a higher molecular weight, generally is more effective in preventing excessive fluid loss from a cement slurry than a xe2x80x9csmallxe2x80x9d polymer, or a polymer with a lower molecular weight. However, large polymers have a negative impact on the properties of the cement slurry. The most common problem associated with large polymers as additives is an unwanted and deleterious increase in viscosity of the cement slurry.
Cement fluid loss additives are needed which prevent excessive fluid loss and which also impart little or no added viscosity to the cement slurry.
The present invention provides a method of making a cement fluid loss additive comprising providing oxygenated water comprising a dissolved oxygen content, mixing a quantity of the oxygenated water with acrylamide monomers to form a reaction mixture, and subjecting the reaction mixture to conditions effective to induce polymerization of the monomers to produce polymers. The dissolved oxygen content is effective to produce polymers which will maintain a cement slurry at an effective viscosity. The conditions are effective to produce polymers which will maintain effective fluid loss control.
The present invention provides a novel method for making improved cement fluid loss additives comprising polymers of acrylamide having desirable viscosity and fluid loss control properties. The method combines (1) the use of oxygenated make-up water to dissolve the acrylamide monomers, their derivatives or combinations thereof, with (2) a second xe2x80x9cconditionxe2x80x9d or set of conditions. Preferably, 2-acrylamido-2-methylpropanesulfonic acid (xe2x80x9cAMPSxe2x80x9d) monomers are used to form a reaction mixture for polymerization of the monomers with other acrylamide monomers, with (2) a second xe2x80x9cconditionxe2x80x9d or set of conditions. The second xe2x80x9cconditionxe2x80x9d or set of conditions is either (a) using of two heating stages to induce polymerization, or (b) adding a small amount of acrylic acid to the reaction mixture before exposing the reaction mixture to a temperature sufficient to polymerize the monomers. When using two heating stages to induce polymerization, the two heating stages involve (i) an initial, lower temperature stage followed by (ii) a higher temperature stage.
Oxygenated water is used to prepare the reaction mixture. The concentration of dissolved oxygen (DO) in the reaction mixture determines the viscosity of the final product. Hereafter, the phrases xe2x80x9coxygen concentrationxe2x80x9d or xe2x80x9coxygen contentxe2x80x9d refer to the dissolved oxygen concentration or the dissolved oxygen content. In reaction mixtures where the starting dissolved oxygen concentration is below about 1000 ppb, polymerization is observed during the initial heating period. While the resulting polymers do function as fluid loss additives, the polymers are excessively viscous, giving Brookfield uncorrected readings of greater than about 80,000. Desirable viscosities for the polymer solution are from about 1000 to about 40,000 units in Brookfield uncorrected readings, preferably about 1500-10,000 units.
When dissolved oxygen concentrations in the reaction mixture are maintained between about 2000 ppb and about 4000 ppb, no noticeable viscosity increases are observed during an initial 3 hour heating period at about 40xc2x0 C., Reaction mixtures which contain saturated levels of dissolved oxygen, 7000 ppb to 8000 ppb, do not significantly increase in viscosity upon heating even to about 90xc2x0 C.; however, the resulting products do not provide adequate fluid loss control properties.
Without limiting the invention to any particular mechanism of action, the dissolved oxygen in the make-up water appears to inhibit the polymerization of the AMPS and acrylamide monomers. Increasing the concentration of dissolved oxygen in the reaction mixture appears to prevent the formation of excessively large copolymers. Reaction charges using water with dissolved oxygen concentrations below about 1000 ppb permit significant polymerization during the initial heating period. This results in higher molecular weight polymers, and excessive viscosity of the cement slurry. Reaction charges using water with a high concentration of dissolved oxygenxe2x80x94at saturation values, or approximately 8000 ppg oxygenxe2x80x94produce products and cement slurries with a low viscosity. However, where the dissolved oxygen is at saturation level, the polymers do not prevent fluid lossxe2x80x94presumably due to the relatively small molecular size of the copolymer. The small molecular size of the polymer allows fluid to pass more easily between the solid particles of the slurry. Using the API-RP-10B test, only fluid losses of about 50 mL or less are considered acceptable.
Water having a known and stable variety of dissolved oxygen contents is available from a variety of sources. Suitable sources of water for deoxygenation include, but are not necessarily limited to tap water, distilled water, and deionized water. Suitable water sources may be deoxygenated using known methods including, but not necessarily limited to purging, use of oxygen scavengers, and displacement and heating to a specified temperature.
In order to achieve a make-up water for the reaction having a desired dissolved oxygen content, it is preferable to downwardly adjust the dissolved oxygen content of water that previously has been oxygenated to a higher level and used to dissolve the AMPS. For example, the dissolved oxygen content of highly oxygenated water can be reduced using time limited delivery of a specific flow rate of an inert gas, such as nitrogen gas. Or, water having different known dissolved oxygen contents may be mixed to achieve a desired dissolved oxygen content. For example, water having a dissolved oxygen content of about 2000 ppb may be prepared by mixing about 1 part water having a relatively high dissolved oxygen content of about 8000 ppb with about 3 parts water having a relatively low dissolved oxygen content of about 100 ppb. The final dissolved oxygen content may be verified using an oxygen meter.
An acrylamide comonomer that may be used in the reaction mixture may be commercially available xe2x80x9cliquidxe2x80x9d acrylamide, which typically contains about 50% solids, or reagent grade acrylamide. A preferred acrylamide is ACRYL-50, a liquid acrylamide product which is available from AmResco, Inc., Solon, Ohio.
Preferred dissolved oxygen contents for most acrylamide monomer combinations are from about 2000 ppb to about 4000 ppb. The preferred dissolved oxygen content may vary with the type of monomers used. For example, Lubrizol Corporation, Wickliffe, Ohio, supplies two commercial grades of solid 2-acrylamido-2-methylpropanesulfonic acid (xe2x80x9cAMPSxe2x80x9d) monomersxe2x80x94AMPS 2404 and AMPS 2401. These two monomers differ by an impurity index that relates to the concentration of polymerization inhibitorsxe2x80x94inhibitors that affect molecular weight maxima. AMPS 2404 provides polymers with maximum molecular weight and minimum size distribution. AMPS 2401 provides products with maximum size distribution, but does so at the expense of molecular weight of the polymer.
The differences appear to be associated with polymerization inhibitors found in the two AMPS monomers. Using AMPS 2404, the dissolved oxygen concentration preferably is lower using liquid acrylamide solutions than when reagent grade acrylamide is used. A similar decrease in dissolved oxygen content is preferred when AMPS 2401 and reagent grade acrylamide are used. Products made with AMPS 2404 and reagent grade acrylamide preferably are prepared using make-up water with a DO content of about 4000 ppb. Products made with AMPS 2401 and reagent grade acrylamide preferably are prepared using water with a DO content of about 2000 ppb.
In the examples, polymerization reactions with liquid acrylamide were carried out only with AMPS 2404. Using AMPS 2404 and the liquid acrylamide solution ACRYL-50, dissolved oxygen concentrations were lowered from about 4000 ppb to about 2000 ppb in order to obtain products having desirable properties. Most likely, a mixture of AMPS 2401 and liquid acrylamide also will require a reduction in dissolved oxygen concentration in order to obtain products having desirable properties, possibly even to 1000 ppb dissolved oxygen or lower.
Products obtained using AMPS 2401 produced cement slurries having acceptably low initial viscosities, and fluid-loss data showed adequate performance. However, the volume of fluid loss was significantly higher than that obtained for products synthesized from AMPS 2404: 30 mL for 2401 vs. 5 mL for 2404. Because of this, and based solely on the criterion of fluid-loss control, AMPS 2404 is a preferred comonomer.
In a preferred embodiment, oxygenated water and preferably equimolar amounts of a first amount of acrylamide monomers and a second amount of AMPS monomers are charged to the reactor. Preferably, the oxygenated water used to dissolve the monomers has a relatively high dissolved oxygen content, preferably about 8000 ppb. After complete dissolution of the solids, the solution is purged with an inert gas, preferably nitrogen. An initiator system is added under a blanket of the inert gas.
Suitable initiator systems include, but are not necessarily limited to persulfate, thiosulfate, sulfite, bisulfite anions or any combination thereof A preferred initiator system is a combination of sodium persulfate and sodium thiosulfate, preferably dissolved in water. The reaction mixture is heated from about 30xc2x0 C. to about 55xc2x0 C., preferably about 40xc2x0 C., for a time period of from about 1 to about 8 hours, preferably about three hours. The time and temperature of this initial heating period apparently are important to maximize fluid loss control properties.
After the initial xe2x80x9clow temperaturexe2x80x9d heating period, the reaction mixture is heated to a higher temperature effective to induce polymerization of the monomers. Suitable temperatures are from about 60xc2x0 C. to about 100xc2x0 C., preferably about 90xc2x0 C. Suitable times are from about 1 to about 8 hours, preferably about three more hours. An inert gas, preferably nitrogen, may or may not be flowed through the reaction vessel during heating. The second heating period initiates and completes the polymerization reaction. In this embodiment, the two step heating sequence is required in order to obtain a polymer with adequate fluid loss properties.
Polymerization reactions performed without the use of the initial heating period gave resulting solutions which did nor perform in a cement fluid loss test with one exception, which is a second embodiment of the invention. Similar product performance is obtained without the need for the initial, low temperature heating step when a small amount of from about 0.01 wt % to about 10.0 wt %, preferably about 0.1% wt % acrylic acid is added to the reaction mixture before heating. Acrylic acid is available from a variety of commercial sources. After addition of acrylic acid, the reaction mixture may be heated directly to the polymerization temperature, preferably about 90xc2x0 C.